1. Field of the Invention
This invention relates to pneumatic percussion-type drill motors of a type known as downhole hammers for drilling or boring through the earth. The invention is more particularly directed to novel means for preventing operation of the hammer-piston and for exhausting the compressed air or other pneumatic fluid past the anvil-bit when the tool is hung off bottom.
2. Description of the Prior Art
Schindler U.S. Pat. No. 3,503,459 discloses and claims a pneumatic percussion motor in which the compressed air or pneumatic fluid supply passages extend through portions of the casing which functions as a whole as a load-carrying structure, as distinguished from conventional prior art pneumatic motors in which separate, non-load-containing sleeves are provided for forming the fluid-carrying passages. The reciprocating hammer-piston has a central exhaust passage which is aligned with a similar passage extending through the anvil-bit slidably received in the forward end of the casing. Finger valves project from the motor back head and the anvil-bit for alternately entering and sealing the central exhaust passage in the hammer-piston during reciprocation thereof and the hammer-piston also having annular passages which alternately register with ports in the casing to direct the working fluid to the opposite ends of the casing for causing the hammer-piston to reciprocate and beat upon the anvil-bit. This structure has the slight disadvantage that when the motor is supported off bottom with the anvil-bit member in a dropped position there is a tendency for compressed air or other pneumatic fluid to be trapped in the working chamber causing the hammer-piston to beat upon the anvil-bit member. It is desirable therefore to have a structure which permits exhausting of the compressed air or pneumatic fluid when the tool is supported with the anvil-bit member in a dropped position.
Various prior art means have been devised for achieving an exhausting of the compressed air when a tool is supported off bottom. Collier, et al U.S. Pat. No. 3,311,177 discloses a type of percussion drill motor in which both the compressed air and the exhaust are conducted through a central porting stem which extends through the piston and into the exhaust duct in the anvil-bit. The anvil-bit, and particularly the exhaust duct therein, is specially configured to provide for exhausting of the forward end of the motor chamber when the tool is suspended off bottom. However, this arrangement cannot be utilized in the percussion motor of the type shown in Schindler U.S. Pat. No. 3,503,459 in which the central porting stem is omitted. In other prior art devices, a centrally bored hammer piston, in its abnormally advanced hanging position, uncovers a blowing port whereby compressed air is bypassed through the anvil and bit.
In Schindler, et al U.S. Pat. No. 3,595,323 there is disclosed a percussion drilling tool of the type shown in Schindler U.S. Pat. No. 3,503,459, in which the working chamber of the tool is vented by means of recesses and passages in the anvil-bit and its mounting chuck or driver sub and/or in the casing, which passages are interconnected only when the tool is blowing and which are disconnected in the normal operating position of the anvil. This tool requires special configurations of parts to provide the interconnecting recesses and passages to provide the desired exhaust of compressed air from the working chamber when the tool is raised off bottom and the anvil-bit is in the dropped position.